1. Field of the Invention
The present invention relates to a wire bonding method for connecting a first bonding point and a second bonding point through a wire, a wire loop having a certain shape and a semiconductor device having such a wire loop incorporated therein.
2. Description of the Related Art
Conventionally, in a process of fabricating a semiconductor device, as shown in FIG. 9A or 9B, wire bonding for connecting a pad 2a or a first bonding point A of a semiconductor chip 2 attached to a lead frame 1 and a lead 1a or a second bonding point Z of the lead frame 1 through a wire 3 has been carried out. Typically, loop shapes of the wire 3 connecting the first and second bonding points A and Z include a trapezoidal shape and a triangular shape shown in FIGS. 9A and 9B, respectively, as disclosed, for example, in U.S. Pat. No. 6,036,080 or Japanese Patent Application Laid-Open Publication No. 2000-277558.
The wire loop having a trapezoidal shape shown in FIG. 9A is formed by a sequence of steps as shown in FIG. 10. First, in step (a) of FIG. 10, a capillary 4 through which the wire 3 passes is lowered and a ball 30 which has been formed at a tip end of the wire 3 is bonded to the pad 2a of the chip 2 or first bonding point A. Then, in step (b) of FIG. 10, the capillary 4 is vertically raised to a point B while the wire 3 is supplied. Thereafter, in step (c) of FIG. 10, the capillary 4 is horizontally moved to a point C in a direction opposite from the second bonding point Z.
In general, such an operation of the capillary 4 to be moved in the direction opposite from the second bonding point Z is referred to as a “reverse operation”. As a result, the portion of the wire 3 between the points A and C is formed to be inclined and the wire 3 is formed at an upper end of the inclined portion thereof with a bend 3a by a lower end of the capillary 4. The portion of the wire 3 between the points A and C thus supplied corresponds to the height of a neck portion H (or a portion of the wire 3 between the pad 2a and the bend 3a) and will constitute the neck portion H.
Subsequently, in step (d) of FIG. 10, the capillary 4 is vertically raised to a point D while the wire 3 is supplied. Then, in step (e) of FIG. 10, the reverse operation of the capillary 4 is performed again, i.e. the capillary 4 is horizontally moved to a point E in the direction opposite from the second bonding point Z. As the result of this reverse operation, the wire 3 has another inclined portion formed to be extending between the points C and E, and a bend 3b is formed in an upper end of this inclined portion of the wire 3.
This inclined portion of the wire 3 thus supplied will constitute an upper base portion L (or a portion of the wire 3 between the bends 3a and 3b) of the wire loop having a trapezoidal shape shown in FIG. 9A. Thereafter, in step (f) of FIG. 10, the capillary 4 is vertically raised to a point F so that the wire 3 is supplied by a length corresponding to a long inclined portion S (or a portion of the wire 3 between the bend 3b and the lead 1a) of the wire loop shown in FIG. 9A. Subsequently, the capillary 4 is lowered to the second bonding point Z via positions f1 and f2, so that the wire 3 is bonded to the second bonding point Z or the lead 1a. 
The wire loop having a triangular shape shown in FIG. 9B is formed by a sequence of steps as shown in FIG. 11. Since the wire loop having a triangular shape is not provided with an upper base portion (L) unlike the wire loop having a trapezoidal shape described above, in forming the wire loop of a triangular shape, the second reverse operation in steps (d) and (e) of FIG. 10 is not conducted. Therefore, in this instance, a step that corresponds to the steps (d), (e) and (f except for f1 and f2) of FIG. 10 is carried out only in step (d) of FIG. 11. More particularly, steps (a), (b) and (c) of FIG. 11 are the same as the steps (a), (b) and (c) of FIG. 10, and after the first reverse operation in step (c) of FIG. 11, the capillary 4 is vertically raised to a point F in step (d) of FIG. 11 while the wire 3 is supplied. Subsequently, in step (e) of FIG. 11, the capillary 4 is moved via positions e1 and e2 in a manner similar to that in step (f) of FIG. 10, with the result that the wire 3 is bonded to the second bonding point Z or the lead 1a. 
However, in the above-described techniques, as the wire loop includes the neck portion H having a somewhat large height, the wire loop becomes high and thus is rendered unstable. In addition, in a case where a wire loop is formed without any reverse operation of the capillary in order to make the height of a neck portion H thereof small and the height of the neck portion H is reduced to a certain level or below, the neck portion H is liable to be damaged in drawing or moving the wire 3 to arrange it in place because of the wire 3 vertically extending from the first bonding point A.
Accordingly, various techniques have been proposed in order to solve the above mentioned problems. For example, U.S. Patent Application Publication No. 2004/0104477 or Japanese Patent Laid-Open Publication No. 2004-172477 discloses a wire loop connecting a first bonding point and a second bonding point through a wire, wherein a top portion of a ball bonded to the first bonding point, together with a part of the wire, is crushed.
The wire loop having such a shape can be formed as a wire loop having a low profile which is stable and strong in shape retention. Not only a wire loop having a short wiring distance but also a wire loop having a long wiring distance can be obtained as a stable wire loop having a low profile. In addition, the wire loop thus formed has a strong shape retention which withstands a force or pressure exerted on the wire loop from outside. Therefore, the wire loop has an excellent shock absorbing function against a shock, such as a shock caused by contact of the capillary or emission of an ultrasonic wave during bonding of the wire to the second bonding point, vibration of the wire, an external force generated by flow of a molding material during injection of the molding material and the like, with the result that bending or tilting of the wire and a breakage in the neck portion of the wire loop can be effectively prevented.
However, in the technique disclosed in U.S. Patent Application Publication No. 2004/0104477, since part of the wire and the top of the bonded ball is crushed by the capillary at the first bonding point, there is a possibility that the first bonding point will be damaged. In addition, the wire which has been crushed at the first bonding point may be caused to partly protrude in a direction opposite from the second bonding point. In the case where wiring is finely effected and a direction of the wire loop is slanted with respect to an edge of a semiconductor chip, the protruding part of the wire may come into contact with a wire of the adjacent first bonding point.